Design rules for advanced generation microlithography (i.e., beyond 193 nm immersion lithography and into next generation optics such as e-beam, X-ray, and extreme ultraviolet (EUV) lithography operating at a very short wavelength of 13.4 nm) are trending toward smaller and smaller dimensions, for example, 30 nm and below. In general, depth of focus (DOF) necessarily decreases with higher resolution due to the higher numerical aperture (NA) and therefore resist thickness also decreases commensurate with the smaller and smaller feature sizes. With narrower linewidths and thinner resist films, increasingly sensitive materials which provide improved resolution are needed.
Backbone fluorination of resist polymers, as an alternative to fluorinating the pendant or leaving groups of a polymer, can improve EUV photon absorbing properties in the polymer without altering leaving group or other properties. However, introducing fluorine atoms near a monomer's polymerizable group affects the reactivity and polymerization behavior of the monomer.
Trifluoromethacrylates (TFMA) are monomers that, when polymerized, incorporate trifluoromethyl groups along the polymer backbone. However, such monomers do not undergo homopolymerization, or copolymerization between different trifluoromethacrylates, readily. Copolymerization of trifluoromethacrylates with electron rich monomers, including norbornenes and vinyl ethers, is possible under common polymerization conditions. Polymerization with acyclic vinyl ether comonomers typically generates high molecular weight polymers under standard radical polymerization conditions; such copolymers may not be soluble in casting solvents typical of photoresist formulations. Ito and coworkers, however, (Ito, H.; Okazaki, M.; Miller, D. C. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 1478) have prepared polymers with significantly lower molecular weights by employing cyclic vinyl ethers. However, copolymers of cyclic vinyl ethers with trifluoromethacrylates may have high dissolution rate.
There remains a need for polymers with fluorinated backbones for use in EUV photoresists which have improved sensitivity, as well as high resolution.